1. Field of the Invention
The present invention is related to solid-state electronic heat pumps, commonly referred to as thermoelectric devices. More particularly, the present invention is related to different thermoelectric device configurations and the methods for making the same.
2. Description of Related Art
A thermoelectric device is capable of generating electricity if its two ends are held at different temperatures. Conversely, applied electrical current will induce a temperature differential between its two ends. Thus, a thermoelectric device is a heat pump which transfers heat by electric current. The principles of thermoelectricity are utilized in power generation, thermocouples, and refrigeration.
Current designs of commercially available thermoelectric devices have efficiencies too low to warrant use in cooling applications with high power microcircuits. However, the use of high thermal conductivity materials along with unique applications of multilayer ceramic technology may be adapted to provide thermoelectric devices with enhanced thermal performance. These devices will be better suited for power generation, cooling integrated circuit chips, and other temperature control applications.
Typically, a thermoelectric device contains p-type and n-type semiconducting materials sandwiched between two ceramic plates; an upper and lower faceplate or carrier plate. The faceplates typically have high electrical resistivity and low thermal conductivity. Situated between the faceplates are a number of Peltier couples, formed by joining p-type and n-type semiconductor elements. These couples are arranged in a two-dimensional array, thermally in parallel, and connected by conductors (braze, solder, and the like) so as to be electrically in series. Typically, a device being cooled is placed in thermal contact with the cold faceplate and a heat sink is placed in contact with the hot faceplate.
The efficiency of a thermoelectric device can be expressed in terms of a figure of merit (Z), defined by the equation: EQU Z=S.sup.2.sigma./.kappa.
where,
Z is expressed in units.times.10.sup.3 /K
S is the Seebeck coefficient in .mu.V/.degree.C.
.kappa. is the thermal conductivity in mW/cm-K
.sigma. is the electrical conductivity in (.OMEGA.-cm).sup.-1
Note that a 1.degree. C. difference is equal to 1K, the unit of absolute temperature. From the above equation, it can be ascertained that in order for a material to be efficient for thermoelectric power conversion, it is important to allow charge carriers to diffuse easily across the multiple Peltier couples while maintaining a temperature gradient. That is, there must be a relatively high value for the Seebeck coefficient (S), a high electrical conductivity (.sigma.), and a low thermal conductivity (.kappa.). When two dissimilar metals (conductors) or semiconductors having different Seebeck potential or Fermi energy levels are in contact at each end, a voltage is obtained if the ends are at different temperatures. This is known as the Seebeck effect. The Seebeck effect is the principle behind thermocouples and power generation devices. In thermocouples, voltages in the millivolt range are typically measured for temperature differences of a few hundred degrees Celsius. If a large number of these junctions are arranged thermally in parallel, electrically in series, and the two ends are held at different temperatures, electrical power at useful voltage is generated.
In contrast, for the Peltier effect, if one applies a current, a temperature difference is then realized. The Peltier effect causes absorption or liberation of heat when current flows across the junction of two dissimilar materials. As electrons flow from a p-type semiconductor to an n-type semiconductor an energy gap or "hurdle" is traversed. Thermal energy is absorbed as electrons overcome this energy hurdle and this junction is cooled. Conversely, as electrons flow from an n-type semiconductor to a p-type semiconductor electrons "fall" down the energy gap and thus release heat. This release will locally heat the junction.
Employing the latest advances in multilayer ceramic technology to thermoelectric device design provides benefits that enable the maximization of heat transfer, provides a device which is physically stronger, incorporates a higher density of interconnects, better protects against exposure to the elements, and enables the thermoelectric devices to be incorporated directly into an integrated circuit package. These advances will provide thermoelectric devices far superior to those in the prior art.
In U.S. Pat. No. 4,946,511 issued to Shiloh et al. on Aug. 7, 1990, entitled "THERMOELECTRIC DEVICES", a thermoelectric device comprising an array of thermoelectric rods of two different materials is taught, where the rods have an alternating arrangement between two carrier plates with each carrier plate being a plurality of discrete conductive metal junctions. In Shiloh, each carrier plate has a nickel plated copper junction on its surface for electrical connection to the thermoelectric rods. Thus, the electrical junctions remain exposed to the environment.
Similarly, in U.S. Pat. No. 4,465,895, issued to Verner, et al., on Aug. 14, 1984, entitled, "THERMOELECTRIC DEVICES HAVING IMPROVED ELEMENTS AND ELEMENT INTERCONNECTS AND METHOD OF MAKING SAME", electrical connection straps are attached to alternating semiconductor rods. However, in one embodiment of Verner, a ceramic potting compound fills the voids between the alternating elements. The compound insulates the elements and protects them from contamination. Although the conductive straps of this design are insulated from environmental effects, they still remain situated on the external surface of the carrier plates.
In U.S. Pat. No. 5,637,921 issued to Burward-Hoy on Jun. 10, 1997, entitled, "SUB-AMBIENT TEMPERATURE ELECTRONIC PACKAGE", two thermoelectric devices are arranged in series to cool an integrated circuit chip. However, the devices remain as separate units, attached to a heat sink or to a package.
In U.S. Pat. No. 4,402,185 issued to Perchak on Sep. 6, 1983, entitled, "THERMOELECTRIC (PELTIER EFFECT) HOT/COLD SOCKET FOR PACKAGED I.C. MICROPROBING", a heat pumping apparatus is described for cooling an IC chip. Two stages of thermoelectric devices are used. However, Perchak teaches the IC chip to be first thermally connected to an aluminum block.
In U.S. Pat. No. 5,032,897 issued to Mansuria et al., on Jul. 16, 1991, entitled, "INTEGRATED THERMOELECTRIC COOLING", the electronic package is adhered to a thermoelectric device, which is then attached to an IC chip. The cold plate of the thermoelectric device is attached directly to the chip, and any consideration for I/O must be done on the opposite side of the chip. Thus, any thermal energy generated by I.sup.2 R heating of lines in the ceramic package will not be drawn away by this thermoelectric device. Consequently, there remains a need to incorporate multilayer ceramic technology in the design of thermoelectric devices in order to fully realize the greatest potential of thermal efficiency.
In U.S. Pat. No. 4,983,225 issued to Rowe on Jan. 8, 1991, entitled, "PROCESS OF MANUFACTURING MINIATURE THERMOELECTRIC CONVERTERS," a method of manufacturing thermoelectric devices is taught. Alternative ion implantation of either n-type or p-type semiconducting materials is used, resulting in a series of Peltier couples being laid out on the surface of a semi-insulating material. This invention, however, is unable to use both high and low thermal conductivity substrates within the device. Furthermore, the ability to create a dense stack of couples in the dimension normal to the substrate is limited by the thickness of the substrate, which must be thick enough to have considerable handling strength.
In U.S. Pat. No. 4,363,928, issued to Wilson on Dec. 14, 1982, entitled, "THERMOELECTRIC GENERATOR PANEL AND COOLING DEVICE THEREFOR," the means of fashioning a thermoelectric device by printing inks of different metals on the surface of a ceramic substrate is taught. This method, however, is limited in the density of Peltier couples that can be provided in the direction normal to the surface of the plane, as the substrates must be stacked on top of each other and thick enough to provide sufficient handling strength. Also, each couple is exposed on two or three sides, providing for inefficient thermal conduction and exposure to condensation from the atmosphere. Lastly, the thermal conductivity of the substrate between the hot and cold side remains uncontrolled.
In U.S. Pat. No. 5,156,688 issued to Buhler, et al., on Oct. 20, 1992, entitled, "THERMOELECTRIC DEVICE," a method of manufacturing a thermoelectric device for power generation is taught, with metal junctions buried between two layers of oxide on a semiconductor substrate. However, the hot and cold sides of the device are necessarily in close proximity of each other. Additionally, the Peltier couples in this device must be formed by micro-circuit fabrication processes.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a thermoelectric device having improved thermoelectric characteristics, and method for making the same.
It is another object of the present invention to provide a thermoelectric array and method for the manufacture of the same having design improvements that make use of multilayer ceramic technology.
A further object of the invention is to maximize the heat flow from the thermoelectric device to the thermal sink.
Another object of the invention is to maximize the heat flow from the object to be cooled to the thermoelectric device.
Yet another object of the invention is to minimize the heat flow between the hot and cold ends of the thermoelectric device.
A further object of the present invention is to provide a thermoelectric device having improved conduction at the electrical and thermal junctions.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.